Studying Heavy Flavor Production at RHIC via Open Charm Measurements at STAR

1. Studying Heavy Flavor Production at RHIC via Open Charm Measurements at STAR Stephen Baumgart for the STAR Collaboration, Yale University

2. Outline • Theoretical Motivation • Detecting Open Charm • Hadronic Reconstruction of D0 • Semi-leptonic Measurement in Cu+Cu • Reconstruction of the Ds in Cu+Cu • Conclusions and Outlook

3. Open Charm Production • Prediction of Charm Cross-Section in p+p from perturbative Quantum Chromodynamics (pQCD) (NLO/FONLL) Ref: M. Cacciari, P. Nason, R. Vogt, Phys. Rev. Lett. 95, 122001 (2005) • Charm produced during initial gluon fusion. • We expect the cross-section to scale with the number of binary collisions • Charm produced before thermalization. • Charm is a good probe of the medium.

4. Prediction of Charm Cross-Section • Method 1: • use dpt slices, then integrate final result • treat charm as active flavor • FONLL Calculation Charm Cross Section Predicted for 200 GeV Collisions: • Method 2: • calculate on full pt range in one step • treat charm as NOT an active flavor (heavy quark considered massive) • NLO Calculation Charm Cross Section Predicted for 200 GeV Collisions: • Ref: R. Vogt, arXiv:0709.2531v1 [hep-ph]

5. Medium Effects • Nuclear modification factor: • Suppression for heavy quarks was predicted to be not as large as that for light quarks (dead cone effect). Ref.: Yu. L Dokshitzer and D.E. Kharzeev, Phys. Lett. B 519 199-206 (2001) • But suppression was measured to be the SAME for both light and heavy flavors. Ref.: B. I. Abelev et al. (STAR),Phys. Rev. Lett. 98 192301 (2007)

6. Measuring Heavy Flavor Electrons detected in the Time Projection Chamber (TPC) can be identified and triggered on by using their energy deposited in the Barrel Electromagnetic Calorimeter (BEMC). The secondary vertex can be located by using the Silicon Vertex Tracker (SVT) or the future Heavy Flavor Tracker (HFT). Charm or beauty is created early in the evolution of the Quark Gluon Plasma, generally from gluon fusion. Full hadronic reconstruction done by using the Time Projection Chamber (TPC)

7. Invariant Mass Reconstruction (Cu+Cu) Kaon Tracks Pion Tracks Unused Tracks Combinatorial Technique Rotational Background Subtraction or Event Mixing Background Subtraction py p +residual background subtraction Momentum and dE/dx cuts used px 5 degree rotations 13 rotations

8. D0 in 200 GeV Cu+Cu Collisions

9. Residual Background Data • Misidentified Resonances (shown in plot) • Collective Flow • Jets • Non-conservation of energy/momentum Signal (K0*) Rotational Background subtraction Signal on top of Residuals (sim.) Residual Background from mis-IDed resonances Signal Only (sim.) Possible Causes:

10. Sources of Systematic Error 0.3 <= pt < 1.3 GeV Full pt range 1.3 <= pt < 2.3 GeV 2.3 <= pt < 3.3 GeV • Double counting • Background Subtraction Method (Rotation vs. Mixing) • Differing Fit Limits (Shown by width of peaks in above plots) • dE/dx Calibration Error

11. D0 +D0 Spectra in 200 GeV Cu+Cu Collisions |y| < 1.0 Fitting Function:

12. Charm Cross-Section number of binary collisions p+p inelastic cross section conversion to full rapidity (using PYTHIA simulation, ver. 6.152) ratio from e+e- collider data *Systematic error evaluation for dN/dy in progress.

13. Checking Binary Scaling • By comparing all systems (d+Au, Cu+Cu, and Au+Au), we check for binary scaling. • New data may allow us to improve on these measurements.

14. Binary Scaling of Cross-Section • NLO Ref: R. Vogt, arXiv:0709.2531v1 [hep-ph] STAR: J. Adams et al. Phys. Rev. Lett 94, 062301 (2005) S. Baumgart, arXiv:nucl-ex/0709.4223 B. I. Abelev, et al,, arXiv:nucl-ex/0805.0364 *Systematic error evaluation for STAR Cu+Cu in progress. PHENIX: S. Adler, et al. Phys. Rev. Lett. 94 082301 (2005) S. Adler, et al. Phys. Rev. Lett. 97 252002 (2006)

15. Non-photonic Electrons in Cu+Cu • STAR can also measure heavy flavor through its semi-leptonic decays channels. • Photonic electrons are cut out via invariant mass cuts. • The vast majority of high-pt non-photonic electrons are the product of charm and beauty decays. • The resultant electron spectra can be used to find RAA.

16. Open Charm RAA in Cu+Cu RAA for non-photonic e± • The preliminary single electron RAA measurement in STAR’s Cu+Cu data set is consistent with previous STAR and PHENIX measurements at similar multiplicities. • This shows that heavy flavor is suppressed like light flavor in 200 GeV Cu+Cu Collisions. STAR: PRL 98 (2007) 192301 PHENIX: PRL 98 (2007) 172301

17. Ds Search Motivation • D/Ds ratio predicted by stat. hadronization model. This can be tested by STAR. • The Ds also contributes to the total charm cross-section. Ref.: I. Kuznetsova and J. Rafelski, Eur. Phys. J. C 51, 113-133 (2007)

18. Ds Reconstruction Technique To Detectors Finding the secondary vertex using the Silicon Vertex Tracker (SVT) allows one to use geometric cuts to identify particles with decay lengthsfrom ~100 mm to 10s of centimeters. K+ K- p+ f (negligible decay length) Secondary Vertex Ds+ (ct = 149.9 mm) To find Ds mesons, I take only decays with decay lengths between 100 and 400 mm. Primary Vertex

19. Phi Reconstruction STAR Preliminary F mesons with displaced vertices

20. Ds in Cu+Cu Weak ~3 sigma Ds+ peak found from reconstruction of Ds +gfp+g K+K-p+ STAR Preliminary No Ds- found. Pythia prediction: STAR Preliminary

21. Conclusions • STAR’s open charm cross-sections are consistent with binary scaling. • STAR’s cross-sections sit near the upper limit of pQCD predictions. • The non-photonic electron RAA in Cu+Cu collisions shows the same amount of suppression that is seen in Au+Au collisions of similar Npart. • There are hints of a charge asymmetry for Ds mesons; however, more data is needed.

22. Outlook • D0 searches (with and w/o SVT) in progress in Run 7 Au+Au and in Run 8 d+Au (low material budget run). • Active Ds searches in Run 7 Au+Au as well as Run 8 d+Au. • Further work will be done with using the SVT to study D0 and Ds spectra.